Both my iron filter and water softener use the old style Autotrol valves actuated by cams. At the ends of the cycles, the valves slam shut creating significant water hammer. The backwash GPM on the iron filter is much higher than the softener and so produces the most water hammer. Unfortunately the poor quality of the water between the precipitation tank and the media tank means that a water hammer arrestor would likely clog up with iron in no time. I could unthread it periodically to clean it out and snake out the line back to the tank at the same time.

My concern is the iron buildup inside the tank or bag. Does the bag in the ST5 hold the air or the water? I really don't have the space for the tank but I do have room for a small arrestor. My softener and iron filter are too tall for my crawlspace so I have their heads poking up through the floor in a closet under the stairs.

The iron filter is programmed to backwash once a week (7 day clock wheel) so there would be 104 hammer events a year. I'm just wondering how many years I might expect it to last.

Three regrets I have with the installation are:
I should have used 1" pipe between the precipitation tank and the filter instead of 3/4". I may still repipe it with 1" hard copper. Would the larger volume of water in the pipe change the force of the water hammer? There would be more mass in motion but it would be moving slower.

I should have kept the run as short as possible. I have about 12 feet of pipe. I could move the precipitation tank a little closer but not a lot. A shorter run would have less mass and therefore less water hammer.

I should have used fewer 90 degree elbows. I used 4 initially but I replaced 2 of them by making sweeps out of 2 45 degree elbows so that I could snake the line. I have 1 more 90 that I can sweep with 2 45's and the last 90 is at the nipple to the softener which I would replace with a Tee to connect the arrestor.

Oh, I have another question related to water hammer. The softener is connected after the iron filter with about 2 feet of pipe. The softener too creates water hammer when it is done backwashing and after the fast rinse/pack. Might the shock wave of the hammer disturb the media pack of the iron filter before it? I know that a flow reversal will unseat the pack as I have done that before.

My concern is the iron buildup inside the tank or bag. Does the bag in the ST5 hold the air or the water? I really don't have the space for the tank but I do have room for a small arrestor. My softener and iron filter are too tall for my crawlspace so I have their heads poking up through the floor in a closet under the stairs.

Air on one side and water on the other I doubt the tank would plug with iron

The iron filter is programmed to backwash once a week (7 day clock wheel) so there would be 104 hammer events a year. I'm just wondering how many years I might expect it to last.

Three regrets I have with the installation are:
I should have used 1" pipe between the precipitation tank and the filter instead of 3/4". I may still repipe it with 1" hard copper. Would the larger volume of water in the pipe change the force of the water hammer? There would be more mass in motion but it would be moving slower.

If you replace it use a couple of unions so you can take it off and flush it out when needed

I should have kept the run as short as possible. I have about 12 feet of pipe. I could move the precipitation tank a little closer but not a lot. A shorter run would have less mass and therefore less water hammer.
You want the arrestor as close to the source of water hammer as possible but there is some leeway, especially with an ST5

I should have used fewer 90 degree elbows. I used 4 initially but I replaced 2 of them by making sweeps out of 2 45 degree elbows so that I could snake the line. I have 1 more 90 that I can sweep with 2 45's and the last 90 is at the nipple to the softener which I would replace with a Tee to connect the arrestor.

Air on one side and water on the other I doubt the tank would plug with iron

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There are three basic designs of captive air tanks. A bag full of air, a bag full of water, and a diaphragm. The bag full of air design is less popular on steel tanks as it leaves more steel exposed. The diaphragm style usually has a tell-tale rolled rib around it.

When it comes to precipitated iron, it gets sticky like toothpaste and coats surfaces. If the bag has air, the iron may built up on the tank wall leaving less room for the bag over time. Same with a diaphragm. The bag of water design probably would be more self-cleaning as the iron may slough off as the bag shrinks and expands. It is all academic however, since I don't really have the room for it.

When I cut into my 3/4" copper pipe to sweep two of the bends, there was 1/4" of iron coating the interior, reducing the pipe to 1/4". This affected the GPM backwash rate. I had cut in a Tee close to the filter a long time ago and attached a full port ballvalve to periodically flush out the line so I thought it would be fine. I was temporarily raising the tank pressure to 80 PSI for the purging but what it was doing, was to reverse the flow through the iron filter media as the stored volume in the house drained back, unsettling the media pack. I quickly learned to follow the purge with a manual backwash and fast rinse/pack to set the media bed right again. For the manual backwash I have to open the bypass on my micronizer so that the pump can produce enough GPM at 80 PSI to give the media a good shakeup.